Convert CONFIG_SYS_BOOTM_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOTM_LEN

As part of this, rework error handling in boot/bootm.c so that we pass
the buffer size to handle_decomp_error as CONFIG_SYS_BOOTM_LEN will not
be available to host tools but we do know the size that we passed to
malloc().

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/pinebook_defconfig b/configs/pinebook_defconfig
index bf070aa..982f681 100644
--- a/configs/pinebook_defconfig
+++ b/configs/pinebook_defconfig
@@ -10,6 +10,7 @@
 CONFIG_R_I2C_ENABLE=y
 CONFIG_SPL_STACK=0x54000
 CONFIG_SYS_PBSIZE=1024
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y
 CONFIG_DM_REGULATOR_FIXED=y