[meson] Re-add build_by_default to embedded targets.

Apparently, build_by_default's default value is dependent on both
the target type (custom_target vs executable) and Meson version.
Even worse, Meson's documentation merely states that it affects "running
plain ninja", and *not* that it also removes the target from the "all"
target.

In #1031 we removed build_by_default, and somehow CI didn't catch it.
This change fixes the resulting #1052.

Signed-off-by: Miguel Young de la Sota <mcyoung@google.com>
diff --git a/sw/device/boot_rom/meson.build b/sw/device/boot_rom/meson.build
index 49af33d..d5edc78 100644
--- a/sw/device/boot_rom/meson.build
+++ b/sw/device/boot_rom/meson.build
@@ -19,6 +19,7 @@
   'boot_rom',
   command: make_embedded_target,
   output: make_embedded_target_outputs,
+  build_by_default: true,
   input: executable(
     'boot_rom',
     sources: [
diff --git a/sw/device/examples/hello_usbdev/meson.build b/sw/device/examples/hello_usbdev/meson.build
index c770c0b..506cfca 100644
--- a/sw/device/examples/hello_usbdev/meson.build
+++ b/sw/device/examples/hello_usbdev/meson.build
@@ -6,6 +6,7 @@
   'hello_usbdev',
   command: make_embedded_target,
   output: make_embedded_target_outputs,
+  build_by_default: true,
   input: executable(
     'hello_usbdev',
     sources: ['hello_usbdev.c'],
diff --git a/sw/device/examples/hello_world/meson.build b/sw/device/examples/hello_world/meson.build
index 33b3b38..dd3dd56 100644
--- a/sw/device/examples/hello_world/meson.build
+++ b/sw/device/examples/hello_world/meson.build
@@ -6,6 +6,7 @@
   'hello_world',  
   command: make_embedded_target,
   output: make_embedded_target_outputs,
+  build_by_default: true,
   input: executable(
     'hello_world',
     sources: ['hello_world.c',],
diff --git a/sw/device/tests/flash_ctrl/meson.build b/sw/device/tests/flash_ctrl/meson.build
index 6b3fe90..aa2f5d1 100644
--- a/sw/device/tests/flash_ctrl/meson.build
+++ b/sw/device/tests/flash_ctrl/meson.build
@@ -6,6 +6,7 @@
   'flash_test',
   command: make_embedded_target,
   output: make_embedded_target_outputs,
+  build_by_default: true,
   input: executable(
     'flash_test',
     sources: ['flash_test.c'],
diff --git a/sw/device/tests/hmac/meson.build b/sw/device/tests/hmac/meson.build
index 3428292..60aacd2 100644
--- a/sw/device/tests/hmac/meson.build
+++ b/sw/device/tests/hmac/meson.build
@@ -6,6 +6,7 @@
   'sha256_test',
   command: make_embedded_target,
   output: make_embedded_target_outputs,
+  build_by_default: true,
   input: executable(
     'sha256_test',
     sources: ['sha256_test.c'],
diff --git a/sw/device/tests/rv_timer/meson.build b/sw/device/tests/rv_timer/meson.build
index 0026172..29328fa 100644
--- a/sw/device/tests/rv_timer/meson.build
+++ b/sw/device/tests/rv_timer/meson.build
@@ -6,6 +6,7 @@
   'rv_timer_test',
   command: make_embedded_target,
   output: make_embedded_target_outputs,
+  build_by_default: true,
   input: executable(
     'rv_timer_test',
     sources: ['rv_timer_test.c'],